U.S. patent application number 10/701972 was filed with the patent office on 2005-05-05 for code individualism and execution protection.
Invention is credited to Chen, Yuqun.
Application Number | 20050097246 10/701972 |
Document ID | / |
Family ID | 34551553 |
Filed Date | 2005-05-05 |
United States Patent
Application |
20050097246 |
Kind Code |
A1 |
Chen, Yuqun |
May 5, 2005 |
Code individualism and execution protection
Abstract
An intrusion-resistant mechanism based on restricted code
segments and code individualization is able to thwart significant
amounts of known and unknown low-level attacks that inject invalid
code, in the form of false data or instructions for execution by a
victim application, by varying the locations of code-containing
segments within a memory space corresponding to an application.
Inventors: |
Chen, Yuqun; (Bellevue,
WA) |
Correspondence
Address: |
LEE & HAYES PLLC
421 W RIVERSIDE AVENUE SUITE 500
SPOKANE
WA
99201
|
Family ID: |
34551553 |
Appl. No.: |
10/701972 |
Filed: |
November 5, 2003 |
Current U.S.
Class: |
710/56 |
Current CPC
Class: |
G06F 21/52 20130101;
G06F 21/14 20130101 |
Class at
Publication: |
710/056 |
International
Class: |
G06F 003/00 |
Claims
What is claimed:
1. A method for securing an application, comprising: defining
containers for at least one code component for the application;
disposing code components in various ones of the containers; and
randomizing locations of the containers.
2. A method according to claim 1, wherein the containers are
contiguous regions of memory.
3. A method according to claim 2, wherein the containers store
individual code components.
4. A method according to claim 2, wherein the containers store
segments of code components.
5. A method according to claim 1, further comprising randomizing an
order of code components in at least one of the respective
containers.
6. A method according to claim 1, further comprising identifying
containers storing executable code components.
7. A method according to claim 1, wherein the containers are
executable.
8. A method according to claim 7, wherein code components for the
application that are not disposed in one of the containers is
non-executable.
9. A method according to claim 1, wherein a virtual page having a
container stored therein is executable.
10. A method according to claim 9, wherein a virtual page without a
container is non-executable.
11. A method of loading an application, comprising: retrieving
metadata regarding components of the application; loading the
components in random locations within a corresponding address
space; and preserving the locations of the components within the
address space.
12. A method according to claim 11, wherein the random locations
include virtual pages.
13. A method according to claim 11, wherein the components include
at least one import table containing an executable and at least one
corresponding dynamic link library (DLL).
14. A method according to claim 13, further comprising identifying
components containing the executable and the at least one
corresponding DLL.
15. A method according to claim 13, further comprising randomizing
locations of the executable and the at least one corresponding DLL
within the at least one import table.
16. A method according to claim 11, further comprising loading the
components in random locations within a corresponding address space
when loading an application.
17. A computer-accessible medium having one or more instructions
for securing a software application, the one or more instructions
causing one or more processors to: retrieve metadata for an
application; inventory code blocks for the application based on the
metadata; and randomly place the code blocks in an address space
for the application.
18. A computer-accessible medium according to claim 17, further
comprising one or more instructions causing the one or more
processors to inventory the randomly placed blocks.
19. A computer-accessible medium according to claim 17, wherein the
code blocks contain at least one of an executable and a DLL.
20. A computer-accessible medium according to claim 17, wherein the
code blocks contain at least one linker table.
21. A computer-accessible medium according to claim 17, further
comprising one or more instructions causing the one or more
processors to randomize an order of components within at least one
of the code blocks.
22. A computer-accessible medium according to claim 17, further
comprising one or more instructions causing the one or more
processors to randomly place the code blocks within the address
space upon every initialization of the application.
23. A computer-accessible medium according to claim 17, wherein the
randomly placed code blocks include only code blocks containing
executable code, as identified by the metadata.
24. A computer-accessible medium having one or more instructions
for securing a software application, the one or more instructions
causing one or more processors to: retrieve metadata for an
application; inventory the import tables for the application; and
randomize the relative positions of the elements in at least one of
the import tables.
25. An application security system, comprising: means for
inventorying code components for an application; means for defining
containers for at least one code component for the application;
means for placing code components in various ones of the
containers; and randomizing locations of the containers.
26. An application security system, comprising: means for
initializing an application; means for retrieving metadata for
components of the application; means for loading the components in
random locations within an address space for the application; and
means for storing the locations of the components within the
address space.
Description
FIELD
[0001] This invention relates to thwarting low-level attacks,
particularly buffer overruns, on software applications.
BACKGROUND
[0002] Attacks in the forms of, e.g., viruses and worms, are among
the most prominent security concerns for software developers.
Software, including operating systems and applications, both
singularly and in combination, are vulnerable to attacks primarily
due to programming errors. These attacks, also referred to as
security breaches, on operating systems and/or applications
typically fall into one of the three categories: low-level attacks,
semantic-level attacks, and configuration-error attacks.
[0003] The first step towards breach-proof software is bug-free
programming. However, a pragmatic perspective on the increasingly
demanding task of programming leads to the realization that
programming bugs are inevitable as applications continue to
increase in sophistication. Static-analysis tools have proven
effective for catching a large number of programming bugs that fit
known patterns, but they still leave the programs vulnerable to
more complex forms of attack. Further, the use of safe programming
languages such as C# eliminates some classes of low-level
vulnerabilities such as a buffer-overrun vulnerability, but
performance is sacrificed by using such languages. That is, such
languages are not conducive to more sophisticated forms of
programming. Furthermore, parts of the run-time system and the
just-in-time compiler, written in traditional C/C++ languages,
still contain bugs.
[0004] Previous efforts at eliminating security breaches on
software include run-time defense mechanisms. For example, bounds
checkers can detect and stop buffer-overrun intrusions, but they
require significant performance overhead. In addition, systems such
as StackGuard and Microsoft.RTM. .NET.RTM. C/C++ compiler can
transform a program to effectively catch buffer overruns on a
stack. However, these systems are unable to catch buffer overruns
in a heap, and their effectiveness against future,
yet-to-be-invented attacks, is dubious. Further still, system-level
techniques intending to prevent direct execution of injected
malicious, or invalid, code are unable to stop indirect execution
of the injected code via carefully-crafted buffer overruns.
SUMMARY
[0005] Methods for thwarting low-level attacks, particularly buffer
overruns, are described herein.
[0006] Such methods include taking an inventory of software code
components for an operating system or an application, defining
segments for at least one of the code components corresponding to
the application, dispersing the at least one code component in
various segments, and randomly dispersing the location of the one
or more code-containing segments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The scope of the present invention will be apparent from the
following detailed description, when taken in conjunction with the
accompanying drawings. The drawings, while indicating embodiments
of the invention, are illustrative only, since various changes and
modifications will become apparent to those skilled in the art from
the following detailed description, in which:
[0008] FIG. 1 shows a client/network system in accordance with
example embodiments;
[0009] FIG. 2 shows an example of an executable import table in
accordance with an example embodiment;
[0010] FIG. 3 shows a memory layout according to an example
embodiment;
[0011] FIG. 4 shows the processing in accordance with an example
embodiment;
[0012] FIG. 5 shows the processing in accordance with another
example embodiment; and
[0013] FIG. 6 illustrates a general computer network environment
which can be used to implement the techniques described herein.
DETAILED DESCRIPTION
[0014] In the example network environment of FIG. 1, multiple
client computing devices 105, 110, 115, and 120, also referred to
as client devices, are coupled to at least one server device 125
via network 100. Network 100 is intended to represent any of a
variety of conventional network topologies and types, which may
include wired and/or wireless networks. Network 100 may further
utilize any of a variety of conventional network protocols,
including public and/or proprietary protocols. Network 100 may
include, for example, the Internet as well as possibly at least
portions of one or more local area networks (LANs).
[0015] Client device 105 may include any of a variety of
conventional computing devices, including a desktop personal
computer (PC), workstations, mainframe computers, Internet
appliances, and gaming consoles. Further client devices associated
with network 100 may include personal digital assistant (PDA) 110,
laptop computer 115, and cellular telephone 120, etc., which may be
in communication with network 100 by a wired and/or wireless link.
Further still, one or more of client devices 105, 110, 115, and 120
may include the same types of devices, or alternatively different
types of devices.
[0016] Server device 125 may provide any of a variety of data
and/or functionality to computing devices 105, 110, 115, and 120.
The data may be publicly available or alternatively restricted,
e.g., restricted to only certain users or available only if the
appropriate fee is paid, etc.
[0017] Server device 125 is at least one of a network server and an
application server, or may be a combination of both. Server device
125 is any device that is the source of content, and client devices
105, 110, 115, and 120 include any devices that receive such
content. Therefore, in a peer-to-peer network, the device that is
the source of the content is referred to as the server device while
the device that receives the content may be referred to as the
client device. Both types of devices are able to load and run
software applications, including operating systems and
applications, in accordance with the example embodiments described
herein.
[0018] At data source 130 or 135, software programs, including
operating systems and applications, are prepared for and/or
provided to any one of server device 125 or client devices 105,
110, 115, and 120 for execution. For the sake of consistency, the
discussion hereafter refers to "applications" which encompass
anyone of, at least, operating systems, programs, and applications,
as known in the art, either singularly or in combination.
Furthermore, the applications are disseminated to server device 125
either off-line, as from data source 130 or on-line, as from data
source 135. Further still, the applications are typically
disseminated to client devices 105, 110, 115, and 120 on-line from
server device 125 or from data source 135. Means and methods for
off-line dissemination thereof are known as well.
[0019] Applications and programs are written in an executable code
(hereafter "executable") in order to be executed by server device
125 or any one of client devices 105, 110, 115, and 120. An
executable contains one or more code component containers which
perform various functions. An executable can make use of specific
functionalities of another executable by loading the second
executable into its virtual address space. This loading process is
called "dynamic linking." To facilitate dynamic linking, the
executable which will be dynamically linked should provide an
"export table" which details the entry points into its executable
code components by which another executable can call. An executable
possessing such features is typically called a "dynamically-linked
library" or a DLL for short. A DLL can be used to perform one or
more functions at runtime for another executable. To automate the
dynamic linking of DLLs, an executable can contain one or more
"import tables," which may be regarded as a linker table describing
specific linkages to other DLLs. As an example, FIG. 2 shows import
table 205 of an executable, which includes hypothetical DLLs,
"CFUNCTION," "EFUNCTION," "BFUNCTION," "AFUNCTION," and "DFUNCTION"
and executable name "BDLL."
[0020] The following description includes references to memory
stacks and memory heaps. Memory stacks refer to portions of memory
that hold local data for particular functions corresponding to
respective DLLs, and memory heaps refer to portions of memory
shared by more than one executable corresponding to a respective
application. That is, stacks are local whereas heaps are
global.
[0021] According to example embodiments described herein, code
execution is disabled on particular data/stacks pages, which are
segments of an application that have been transferred into memory,
thus eliminating the potential for invalid code that is injected
into the heap or onto the stack from being executed. In other
example embodiments, the layout of valid import tables, including
executables and their respective DLLS, is randomized to prevent
attacks from utilizing valid code existing in an application.
[0022] In a first example embodiment for disabling code execution
on data/stack pages, code pages are separated from data and stack
pages. Current operating systems (hereafter "OS") including, e.g.,
Microsoft.RTM. Windows.RTM. operating systems (Windows.RTM.
XP.RTM., 2000.RTM., and future versions thereof), code segments,
data segments, and stack segments are all set to a same range
covering an entire 32-bit address space. In this first embodiment,
a special intrusion-resistant execution mode (IREX) is added to the
OS to protect certain security-sensitive applications.
[0023] FIG. 3 shows an example memory layout of an IREX application
for a first user address space 305 and a second user address space
320. In both user spaces, an application loading module may dispose
respective user code segments 310 and 325 at any position in
respective user address spaces 305 and 320. Further, the
application loading module may utilize the entire user address
space to position the respective stack segments containing user
data 315 and 330. Thus, FIG. 3 shows that, in accordance with an
example embodiment, the same code segment, including a DLL or
static data, may be located at different address locations in
different address spaces. As a result, by rendering the virtual
code pages respectively corresponding to user address spaces 305
and 320 to be "read-only executable" pages, a buffer-overrun
attempt is unable to modify the code segment. For example, for an
AMD.RTM. 64-bit architecture, an Intel.RTM. 64-bit architecture, or
a 32-bit X86 architecture with Physical Address Extension (PAE),
instead of producing a special code segment, it is sufficient to
mark data and stack virtual pages as being "non-executable," and
marking the code pages as being "executable" and "read-only."
[0024] In accordance with the example embodiment of FIG. 3, a host
OS rearranges the layout of executable code in the user code
segment upon application installation and/or its run-time
initialization. Therefore, it is difficult for an attacker
(hereafter "hacker") to determine the exact locations of particular
instructions in the application, hence defeating hacking efforts
that attempt to make use of a known instruction sequence in the
application itself to execute using invalid data that has been
injected into the application. The layout rearrangement produces a
functionally equivalent version of the program and can be performed
on any of server device 125 or client devices 105, 110, 115, and
120 (see FIG. 1).
[0025] Thus, FIG. 3, above, refers to a first form of program
individualism, referred to as "block rearrangement." The processing
for block rearrangement, in accordance with the example of FIG. 3,
is shown in FIG. 4. The process implemented during the installation
begins with initialization 405, which includes an inventorying
process. The inventorying includes reading 410 metadata
identifying, at least, DLLs to be loaded during execution as well
as data regarding the size of code segments corresponding to the
application including import tables and other code segments. That
is, the metadata provides threshold information regarding the block
layout in the application. The metadata may be incorporated in the
OS and/or application, or the metadata may accompany the OS and/or
application in another format known in the art. The code
rearrangement 415 randomizes the location of code-containing
segments in a memory utilizing a random generator that generates a
random value within the appropriate address space. That is, as an
example, at least one of the import tables corresponding to the
application is disposed at a random location within the memory
space for the application upon the loading of the application.
[0026] Further to such implementation, the DLLs specified by an
import table may be randomly rearranged, either in combination with
the block rearrangement described above or as an individual
security measure. In such implementation, code rearrangement 415
further includes randomly rearranging the hypothetical DLLs,
"CFUNCTION," "EFUNCTION," "BFUNCTION," "BDLL," "AFUNCTION," and
"DFUNCTION" in the example import table of FIG. 2. To re-arrange
basic function blocks (or DLLs) in an application, the blocks are
to be identified, rearranged, and the addresses of the rearranged
blocks, or code component containers, need to be updated.
[0027] The block rearrangement may be implemented in either an
application-specific manner or a system-wide manner, during the
installation of at least one of the OS and application. That is, an
extra layer of security may be gained by implementing block
rearrangement, first, when loading an OS and, secondly, when
loading the application. However, in order for the same executable
or DLL image to be shared among multiple processes, the
block-rearrangement operation should be performed only once on each
client device or server device. Still, the implementation may be
re-applied after a system (or application) shutdown at a periodic
interval to increase the protection strength.
[0028] The foregoing example embodiment for block rearrangement may
further randomize executables and corresponding DLLs of an
application by loading them into random locations, in accordance
with different spatial orders in a code segment. Code segments may
also be referred to as code containers, and are intended to store
code components, which include, e.g., executables and their
corresponding DLLs. The initial temporal order in which the
executables and corresponding DLLs are loaded is preserved,
however. The same DLL should be mapped into the same page offset
across different processes. The offset need not be zero, as long as
it is cache-aligned.
[0029] To further reduce any likelihood that a hacker is able to
correctly guess the starting location of a particular DLL, a code
segment is allocable to any portion of an address space
corresponding to the IREX process. For example, on a 32-bit server
device or client device with a 2 GB user address space, the code
segment may be allocated to the first GB of virtual space. Thus, if
the code segment is 100 MB and the DLLs are mapped at a cache-line
alignment (32 bytes), there are approximately 30,277,632 possible
locations a specific instruction can be at.
[0030] Since a particular DLL may be loaded at different
cache-aligned virtual addresses in different IREX processes,
consideration is given to the fact that any reduction in
consumption of physical memory requires a DLL to be shared. That
is, code in a DLL may require access to static data that is also
allocated during the load process. Therefore, according to an
example embodiment, memory references to the static data section
are made using absolute addressing and fixed up during the DLL
loading process. Sharing an IREX DLL across multiple processes
require that its static data section be at the same relative
distance to the DLL code in all these processes.
[0031] The example embodiment further includes at least two sample
solutions for avoiding collisions between static data corresponding
to a DLL and a heap page that has already been allocated. First, if
an IREX loader chooses from a large virtual address space, e.g., 1
GB, to allocate a code segment, the IREX loader can pick a range of
virtual addresses to map the code segment such that virtual
addresses for mapping the static data sections corresponding to a
DLL are all available. Second, access to the static data section
can be done via indirection. That is, the loader may allocate a
read-only region at a known location, for example, page 0, for
storing virtual addresses of each static data section corresponding
to a DLL. Within each DLL, instead of directly accessing a location
in the static section, a read is first made to fetch the actual
location of the static section, and then the access is made via an
offset into the data section. This location can be loaded into a
free code segment at the beginning of a function if such code
segment is available. In order for the same DLL to be mapped at
different virtual addresses across multiple processes, the
addressing within a DLL must be relative.
[0032] According to another example embodiment, load-time
re-ordering shuffles the memory locations at which the import table
including executable and corresponding DLLs is loaded. Such
randomized loading is implemented upon each loading operation at a
respective server or client device. This implementation results in
unpredictable locations for any function sequences that a hacker
wishes to exploit. The operation is inexpensive in that the loader
merely determines where to load each executable and respectively
corresponding DLL, and is performed independently for every IREX
process.
[0033] More specifically, when an IREX application is shipped to
any one of server device 125 or client devices 105, 110, 115, and
120 (see FIG. 1), it comes with, either inclusively or as an
attachment, corresponding IREX metadata. The metadata identifies,
at least, all possible DLLs to be loaded during execution as well
as data regarding the size of the code segment. The IREX metadata
is packaged along with the application and is signed by the
application vendor.
[0034] During the installation, as shown in FIG. 5, the modified OS
installer authenticates 505 the IREX signature using protocol known
in the art, unpacks 510 the IREX metadata, and rearranges the
blocks. Such rearrangement may be made in accordance with
instructions included in the metadata. The metadata is then stored
in an internal database of the OS installer in the appropriate one
of server device 125 or client device 105, 110, 115, and 120 for
further use by the (modified) OS loader.
[0035] As set forth above regarding the example embodiment of FIG.
4, alternative embodiments may relocate only the DLLs at random
addresses and still achieve desirable measures of intrusion
resistance. Thus, the block rearrangement implementation
exemplified by FIG. 5 provides an increased measure of intrusion
resistance beyond that of randomizing the location of just the
DLLs.
[0036] Alternate embodiments to the examples of both FIGS. 4 and 5
include performing the block relocation described above prior to
distribution of individualized copies of operating systems or
applications. Such embodiment provides an extra level of security
protection to an application vendor that is concerned about
shipping block information that may aid hackers in
reverse-engineering the code.
[0037] FIG. 6 illustrates a general computer environment 600, which
can be used to implement the techniques described herein. The
computer environment 600 is only one example of a computing
environment and is not intended to suggest any limitation as to the
scope of use or functionality of the computer and network
architectures. Neither should the computer environment 600 be
interpreted as having any dependency or requirement relating to any
one or combination of components illustrated in the example
computer environment 600.
[0038] Computer environment 600 includes a general-purpose
computing device in the form of a computer 602. The components of
computer 602 can include, but are not limited to, one or more
processors or processing units 604, system memory 606, and system
bus 608 that couples various system components including processor
604 to system memory 606.
[0039] System bus 608 represents one or more of any of several
types of bus structures, including a memory bus or memory
controller, a peripheral bus, an accelerated graphics port, and a
processor or local bus using any of a variety of bus architectures.
By way of example, such architectures can include an Industry
Standard Architecture (ISA) bus, a Micro Channel Architecture (MCA)
bus, an Enhanced ISA (EISA) bus, a Video Electronics Standards
Association (VESA) local bus, a Peripheral Component Interconnects
(PCI) bus also known as a Mezzanine bus, a PCI Express bus, a
Universal Serial Bus (USB), a Secure Digital (SD) bus, or an IEEE
1394, i.e., FireWire, bus.
[0040] Computer 602 may include a variety of computer readable
media. Such media can be any available media that is accessible by
computer 602 and includes both volatile and non-volatile media,
removable and non-removable media.
[0041] System memory 606 includes computer readable media in the
form of volatile memory, such as random access memory (RAM) 610;
and/or non-volatile memory, such as read only memory (ROM) 612 or
flash RAM. Basic input/output system (BIOS) 614, containing the
basic routines that help to transfer information between elements
within computer 602, such as during start-up, is stored in ROM 612
or flash RAM. RAM 610 typically contains data and/or program
modules that are immediately accessible to and/or presently
operated on by processing unit 604.
[0042] Computer 602 may also include other removable/non-removable,
volatile/non-volatile computer storage media. By way of example,
FIG. 6 illustrates hard disk drive 616 for reading from and writing
to a non-removable, non-volatile magnetic media (not shown),
magnetic disk drive 618 for reading from and writing to removable,
non-volatile magnetic disk 620 (e.g., a "floppy disk"), and optical
disk drive 622 for reading from and/or writing to a removable,
non-volatile optical disk 624 such as a CD-ROM, DVD-ROM, or other
optical media. Hard disk drive 616, magnetic disk drive 618, and
optical disk drive 622 are each connected to system bus 608 by one
or more data media interfaces 625. Alternatively, hard disk drive
616, magnetic disk drive 618, and optical disk drive 622 can be
connected to the system bus 608 by one or more interfaces (not
shown).
[0043] The disk drives and their associated computer-readable media
provide non-volatile storage of computer readable instructions,
data structures, program modules, and other data for computer 602.
Although the example illustrates a hard disk 616, removable
magnetic disk 620, and removable optical disk 624, it is
appreciated that other types of computer readable media which can
store data that is accessible by a computer, such as magnetic
cassettes or other magnetic storage devices, flash memory cards,
CD-ROM, digital versatile disks (DVD) or other optical storage,
random access memories (RAM), read only memories (ROM),
electrically erasable programmable read-only memory (EEPROM), and
the like, can also be utilized to implement the example computing
system and environment.
[0044] Any number of program modules can be stored on hard disk
616, magnetic disk 620, optical disk 624, ROM 612, and/or RAM 610,
including by way of example, operating system 626, one or more
application programs 628, other program modules 630, and program
data 632. Each of such operating system 626, one or more
application programs 628, other program modules 630, and program
data 632 (or some combination thereof) may implement all or part of
the resident components that support the distributed file
system.
[0045] A user can enter commands and information into computer 602
via input devices such as keyboard 634 and a pointing device 636
(e.g., a "mouse"). Other input devices 638 (not shown specifically)
may include a microphone, joystick, game pad, satellite dish,
serial port, scanner, and/or the like. These and other input
devices are connected to processing unit 604 via input/output
interfaces 640 that are coupled to system bus 608, but may be
connected by other interface and bus structures, such as a parallel
port, game port, or a universal serial bus (USB).
[0046] Monitor 642 or other type of display device can also be
connected to the system bus 608 via an interface, such as video
adapter 644. In addition to monitor 642, other output peripheral
devices can include components such as speakers (not shown) and
printer 646 which can be connected to computer 602 via I/O
interfaces 640.
[0047] Computer 602 can operate in a networked environment using
logical connections to one or more remote computers, such as remote
computing device 648. By way of example, remote computing device
648 can be a PC, portable computer, a server, a router, a network
computer, a peer device or other common network node, and the like.
Remote computing device 648 is illustrated as a portable computer
that can include many or all of the elements and features described
herein relative to computer 602. Alternatively, computer 602 can
operate in a non-networked environment as well.
[0048] Logical connections between computer 602 and remote computer
648 are depicted as a local area network (LAN) 650 and a general
wide area network (WAN) 652. Such networking environments are
commonplace in offices, enterprise-wide computer networks,
intranets, and the Internet.
[0049] When implemented in a LAN networking environment, computer
602 is connected to local network 650 via network interface or
adapter 654. When implemented in a WAN networking environment,
computer 602 typically includes modem 656 or other means for
establishing communications over wide network 652. Modem 656, which
can be internal or external to computer 602, can be connected to
system bus 608 via I/O interfaces 640 or other appropriate
mechanisms. It is to be appreciated that the illustrated network
connections are examples and that other means of establishing at
least one communication link between computers 602 and 648 can be
employed.
[0050] In a networked environment, such as that illustrated with
computing environment 600, program modules depicted relative to
computer 602, or portions thereof, may be stored in a remote memory
storage device. By way of example, remote application programs 658
reside on a memory device of remote computer 648. For purposes of
illustration, applications or programs and other executable program
components such as the operating system are illustrated herein as
discrete blocks, although it is recognized that such programs and
components reside at various times in different storage components
of computing device 602, and are executed by at least one data
processor of the computer.
[0051] Various modules and techniques may be described herein in
the general context of computer-executable instructions, such as
program modules, executed by one or more computers or other
devices. Generally, program modules include routines, programs,
objects, components, data structures, etc. for performing
particular tasks or implement particular abstract data types.
Typically, the functionality of the program modules may be combined
or distributed as desired in various embodiments.
[0052] An implementation of these modules and techniques may be
stored on or transmitted across some form of computer readable
media. Computer readable media can be any available media that can
be accessed by a computer. By way of example, and not limitation,
computer readable media may comprise "computer storage media" and
"communications media."
[0053] "Computer storage media" includes volatile and non-volatile,
removable and non-removable media implemented in any method or
technology for storage of information such as computer readable
instructions, data structures, program modules, or other data.
Computer storage media includes, but is not limited to, RAM, ROM,
EEPROM, flash memory or other memory technology, CD-ROM, digital
versatile disks (DVD) or other optical storage, magnetic cassettes,
magnetic tape, magnetic disk storage or other magnetic storage
devices, or any other medium which can be used to store the desired
information and which can be accessed by a computer.
[0054] "Communication media" typically embodies computer readable
instructions, data structures, program modules, or other data in a
modulated data signal, such as carrier wave or other transport
mechanism. Communication media also includes any information
delivery media. The term "modulated data signal" means a signal
that has one or more of its characteristics set or changed in such
a manner as to encode information in the signal. As a non-limiting
example only, communication media includes wired media such as a
wired network or direct-wired connection, and wireless media such
as acoustic, RF, infrared, and other wireless media. Combinations
of any of the above are also included within the scope of computer
readable media.
[0055] Reference has been made throughout this specification to
"one embodiment," "an embodiment," or "an example embodiment"
meaning that a particular described feature, structure, or
characteristic is included in at least one embodiment of the
present invention. Thus, usage of such phrases may refer to more
than just one embodiment. Furthermore, the described features,
structures, or characteristics may be combined in any suitable
manner in one or more embodiments.
[0056] One skilled in the relevant art may recognize, however, that
the invention may be practiced without one or more of the specific
details, or with other methods, resources, materials, etc. In other
instances, well known structures, resources, or operations have not
been shown or described in detail merely to avoid obscuring aspects
of the invention.
[0057] While example embodiments and applications of the present
invention have been illustrated and described, it is to be
understood that the invention is not limited to the precise
configuration and resources described above. Various modifications,
changes, and variations apparent to those skilled in the art may be
made in the arrangement, operation, and details of the methods and
systems of the present invention disclosed herein without departing
from the scope of the claimed invention.
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